How to define a pointer to a structure
Solution 1
typedef struct contactInfo
{
int number;
char id;
}ContactInfo;
This code defines 2 things:
- a type named
ContactInfo
- a
struct
namedcontactInfo
Notice the difference of the c
and C
!
In your code you are using a mixed combination of both, which is allowed (although confusing IMHO).
If you use the struct
variant you need to explicitly use struct contactInfo
. For the other variant (ContactInfo
) you must to omit the struct
part as it is part of the type definition alteady.
So be careful with both different definitions of your structure. Best would be to only use either one of the variants.
I do not have Visual Studio at hand, but the following (corrected) code compiles with gcc properly without any warnings:
#include<stdlib.h>
typedef struct contactInfo
{
int number;
char id;
}ContactInfo;
void main()
{
ContactInfo nom,*ptr;
ptr=malloc(2*sizeof(ContactInfo));
}
(I left out the not so interesting/unmodified parts of the code)
Solution 2
This:
ptr=(contactInfo*)malloc(2*sizeof(contactInfo));
is wrong, there's no type called contactInfo
.
There's a struct contactInfo
, which is typedef
:d as ContactInfo
. C is case-sensitive (and you must include the struct
unlike how it works in C++).
Also note that the quoted line is better written as:
ptr = malloc(2 * sizeof *ptr);
Since casting the return value of malloc()
is a bad idea in C, I removed the cast. Also, repeating the type is a bad idea since it makes the risk of introducing error greater, so I removed that as well.
Note that sizeof *ptr
means "the size of the the type that ptr
points at" and is a very handy thing.
Comments
-
SLearner almost 2 years
I know this is a very basic problem, but I cannot move forward without it and its not clearly explained elsewhere.
Why is this programming giving me so many errors of undeclared identifier? I have declared it, though.
These are the error i am getting.
Error 2 error C2143: syntax error : missing ';' before 'type' Error 3 error C2065: 'ptr' : undeclared identifier Error 4 error C2065: 'contactInfo' : undeclared identifier Error 5 error C2059: syntax error : ')' Error 15 error C2223: left of '->number' must point to struct/union
and more...
#include<stdio.h> #include<stdlib.h> typedef struct contactInfo { int number; char id; }ContactInfo; void main() { char ch; printf("Do you want to dynamically etc"); scanf("%c",&ch); fflush(stdin); struct contactInfo nom,*ptr; ptr=(contactInfo*)malloc(2*sizeof(contactInfo)); nom.id='c'; nom.number=12; ptr->id=nom.id; ptr->number=nom.number; printf("Number -> %d\n ID -> %c\n",ptr->number,ptr->id); }
-
Veger over 11 years
contactInfo
andContactInfo
are 2 different things. So besides case-sensitivity, this is something to keep in mind! -
SLearner over 11 yearsbut if i don't typecast malloc, will it not return char type?
-
Aniket Inge over 11 yearsI agree, contactInfo is a struct tag, ContactInfo is a typedef of that struct.. if I am right @Veger
-
Veger over 11 yearsThat is right, hence my comment that case-sensitivity is not the only problem here.
-
SLearner over 11 yearsOk so i removed the typecast, but still its showing me the same errors. I am using VS 2010 and have changed the compiler to c\tc i still dont get what's the problem here @unwind . thanks though!
-
SLearner over 11 yearsI tried but its still not working. Is the VS 2010 compiler screwing it up? almost all the errors are pointing to the ptr=(contactInfo*)malloc(2*sizeof(contactInfo)); line also removing the typecast adds more error to the program :/
-
SLearner over 11 yearsi tried and its not working. I think there are other problems as well?
-
Veger over 11 yearsI added a fixed version of your code that compiles properly with gcc to my answer to show the required corrections
-
Paul R over 11 years@akp: see stackoverflow.com/questions/605845/…
-
Paul R over 11 yearsOK - if you fix your answer I'll happily remove the down-vote.